History of Genetics
People have known about inheritance for a long
time.
 Children resemble their parents
 Domestication of animals and plants, selective
breeding for good characteristics
 Sumerian horse breeding records
 Egyptian data palm breeding
 Ability to indentify a person as a member of a
particular family by certain physical traits
Old Ideas
Despite knowing about inheritance in general, a
number of incorrect ideas had to be generated
and overcome before modern genetics could
arise.
1. All life comes from other life. Living organisms
are not spontaneously generated from non-living
material. Big exception: origin of life.
2. Species concept: offspring arise only when two
members of the same species mate. Monstrous
hybrids don’t exist.
More Old Ideas
3. Organisms develop by expressing information
carried in their hereditary material. As opposed
to “preformation”, the idea that in each sperm
(or egg) is a tiny, fully-formed human that
merely grows in size.
4. The environment can’t alter the hereditary
material in a directed fashion. There is no
“inheritance of acquired characteristics”.
Mutations are random events.
More Old Ideas
5. Male and female parents contribute equally to
the offspring.
 ancient Greek idea: male plants a “seed” in
the female “garden”.
 alleged New Guinea belief: sex is not related
to reproduction.
Greek Philosophers
• Theophrastus proposed that male flowers caused
female flowers to ripen
• Hippocrates speculated that “seeds” were produced
by various body parts and transmitted to offspring
at the time of conception
• Aristotle though that male and female semen mixed
at conception
• Aeschylus proposed the male as the parent with the
female as a “nurse for the young life sown within
her”
Different Old Theories explained the
Similarities and Dissimilarities
between Individuals
 Blending theory
The mixture of sperm and egg resulted in progeny that were
a “blend” of two parent’s characteristics.
 Acquired characters inheritance (Jean Baptiste
Lamarck)
Individuals inherit traits are strengthened by their parents
 Pangenesis (Charles Darwin)
The cells excreted gemmules then collected and
concentrated in the reprodutive organ. Fathers and
mother gemmules blended to form an embryo
Different Old Theories explained the
Similarities and Dissimilarities
between Individuals
 Performation and Epigenesis
Organism develop by expressing information carried
in their heredity material
 Cell Theory (Hooke, Leewenhoek, Schleiden,
Schwann, Virchow)
1. All living things are composed of one or more cells
2. Cells are the basic units of structure and function in living
things (Schleiden and Schwann, 1839)
3. New cells are produced from existing cells (Virchow,
1858)
Mid 1800’s Discoveries
• Three major events in the mid-1800’s led
directly to the development of modern genetics.
1859: Charles Darwin publishes The Origin of Species,
which describes the theory of evolution by
natural selection. This theory requires heredity to
work.
1866: Gregor Mendel publishes Experiments in Plant
Hybridization, which lays out the basic theory of
genetics. It is widely ignored until 1900.
1871: Friedrich Miescher isolates “nucleic acid” from
pus cells.
Gregor Mendel (1822-1884)
• Systematically
recorded results of
crosses
• Theorized on nature
of hereditary material
• Postulate mechanism
of transfer of
"Elementen"
governing traits
Mendel’s Work with Peas
a. He selected strains that differed
in particular traits (e.g., smooth
or wrinkled seeds, purple or
white flowers)
b. After making genetic crosses, he
counted the appearance of traits
in the progeny and analyzed the
results mathematically.
c. He concluded that each
organism contains two copies of
each gene, one from each parent,
and that alternative versions of
the genes (alleles) exist
Mendel’s Work with Peas
He deduced that the factors (now called genes) segregate
randomly into gametes (Mendel’s first law, the Principle of
Segregation).
5. The two factors for a particular trait assort independently of
factors controlling other traits (Mendel’s second law, the
Principle of Independent Assortment).
6. An example is seed color in peas:
i. True-breeding plants with yellow seeds (YY) are crossed
with true-breeding plants with green seeds (yy).
ii. The progeny (F1) have yellow seeds, and a heterozygous
genotype (Yy).
iii. When the progeny self-pollinate, the F2 contains 3
yellow:1 green, with genotypic ratios of 1 YY : 2 Yy : 1 yy.
4.
Major Events in the 20th
Century
• 1900: rediscovery of Mendel’s work by Robert Correns,
Hugo de Vries, and Erich von Tschermak .
• 1902: Archibald Garrod discovers that alkaptonuria, a
human disease, has a genetic basis.
• 1904: Gregory Bateson discovers linkage between
genes. Also coins the word “genetics”.
• 1910: Thomas Hunt Morgan proves that genes are
located on the chromosomes (using Drosophila).
• 1918: R. A. Fisher begins the study of quantitative
genetics by partitioning phenotypic variance into a
genetic and an environmental component.
Thomas Hunt Morgan
• Thomas Hunt Morgan: early 1900’s
– Worked at Columbia University; later at
CalTech
– Studied fruit fly eye color, determining
that trait was sex-linked
– Won the Nobel Prize in 1933 for his
work on chromosomes and genetics
Thomas Hunt Morgan
Thomas Hunt Morgan
• By this point, it was known that genetic
material was located on a chromosome
• This genetic material was in discrete units
called genes
• It was NOT known whether the gene was
simply a protein, or whether it was
composed of DNA
More 20th Century Events
• 1926: Hermann J. Muller shows that X-rays
induce mutations.
• 1944: Oswald Avery, Colin MacLeod and
Maclyn McCarty show that DNA can
transform bacteria, demonstrating that
DNA is the hereditary material.
• 1953: James Watson and Francis Crick
determine the structure of the DNA
molecule, which leads directly to
knowledge of how it replicates
More 20th Century Events
• 1966: Marshall Nirenberg solves the
genetic code, showing that 3 DNA bases
code for one amino acid.
• 1972: Stanley Cohen and Herbert Boyer
combine DNA from two different species
in vitro, then transform it into bacterial
cells: first DNA cloning.
• 2001: Sequence of the entire human
genome is announced.
James Watson and
Francis Crick
James Watson and
Francis Crick
• Used wire models to conform with
the measurements that Franklin and
Wilkins had come up with
• Determined the structure to be a
double helix
• Lead to understanding of mutation
and relationship between DNA and
proteins at a molecular level
• 1959 – “Central Dogma”
– DNARNAprotein
Central Dogma of Biology